1. Field of the Invention
This invention relates generally to detectors of electromagnetic radiation and, in particular, to thermopile radiation detectors.
2. Description of the Re
Conventional thermopiles are generally of the two types shown in FIGS. 1 and 2. FIG. 1 illustrates a solid backed thermopile 1 having a substrate 2 that contains a layer 3 comprised of a thermal and electrical insulating material. Overlying the layer 3 is a bi-metallic thermocouple junction 6 comprised of a first metal layer 4 (e.g., Bi) and a second metal layer 5 (e.g., Sb). Disposed over the top of the thermocouple junction 6 is a layer 8 of blackening material that renders the thermocouple metals layers 4 and 5 more absorptive at all wavelengths. In this device incident radiation is directed as indicated by the arrow designated .lambda..
Reference in this regard can be had to a publication by R. W. Astheimer et al., "Solid-Backed Evaporated Thermopile Radiation Detectors", Applied Optics, vol. 3, no. 4, 4/64. As related therein a solid-backed thermocouple radiation detector consists essentially of a pair of metallic junctions deposited onto a heat sink, one junction being in good thermal contact with the sink by conduction while the other is thermally isolated by a thin insulating layer. The area of the active junction is considered to be a blackened portion over the insulator, while the "cold" or reference junction is formed by the opposite ends of the metal strips which are in good conductive thermal contact with the sink.
FIG. 2 illustrates a conventional film-backed thermopile 1' and having an aperture or cavity 9 made within the substrate 2 beneath the thermocouple junction 6. The film-backed thermopile 1' is similar to the solid backed thermopile of FIG. 1, except that an electrically insulating film layer 7 (such as Al.sub.2 O.sub.3 or mylar) replaces the insulating layer 3 of FIG. 1. A layer 8 of blackening material is provided to enhance the radiation absorbing qualities of the metal layers 4 and 5. L1 and L2 generally indicate electrical leads which are provided to read-out the voltage that is generated by the thermocouple junction 6. The approach of FIG. 2 generally increases the electrical response of the thermopile (in volts output per watt of radiation received) over the device of FIG. 1. However, the increase in electrical response is made at the expense of reducing the frequency response.
Reference can also be made to commonly assigned U.S. Pat. No. 3,405,272 (Oct. 8, 1968), entitled "Film Supported Detector with Low Heat Transfer Impedance Path from Cold Junctions to Thermal Sink" by N. B. Stevens et al., and also to commonly assigned U.S. Pat. No. 3,405,271 (Oct. 8, 1968), entitled "Detector Having Radiation Collector Supported on Electrically Insulating Thermally Conducting Film" by N. B. Stevens et al. In both of these commonly assigned U.S. Patents an aluminum oxide film is disposed over surfaces of a cold sink and spans a cavity within the cold sink. The aluminum oxide film supports two thermoelectric materials (Bi and Sb).
Referring again to FIG. 1 and FIG. 2, the blackening material 8, such as gold black or bismuth black, is evaporated onto the sensitive area of the thermopile. This blackening material renders the surface generally at least 80 percent absorptive at all wavelengths, thus increasing the responsivity (in volts/watt) of the device.
It is noted that in FIGS. 1 and 2 only one thermocouple junction is shown. However, in most practical devices there are a number of such junctions (e.g., 15 to 25) in a series connection in order to increase the voltage output for a given radiation input power. For example, if one junction yields 100 .mu.V/.degree. C., 20 junctions would ideally yield 2000 .mu.V/.degree. C.
One significant disadvantage of these conventional approaches is that the thermopile is not spectrally selective, i.e., is not tuned to a specific, relatively narrow range of wavelengths. This is due to the fact that the conventional blackening materials used to form the layer 8 absorb over a wide spectral region, typically from the ultraviolet to the far infrared.
To overcome this problem, and to thus make a thermopile detector that is spectrally selective, it is necessary to position a discrete optical filter between the thermopile detector and the source of radiation to be detected. The optical filter's spectral characteristics are selected so as to pass only the range of wavelengths that are desired to be detected.
As can be appreciated, the use of an external optical filter adds cost, complexity, and mass to the overall radiation detection system.
Furthermore, an external filter is preferably placed at a focal point of the optical system in order to reduce the size of the filter. This requirement further complicates the design of the overall optical system, and may require that an additional focal point be provided for the filter.
Additionally, the requirement to provide an external filter limits a number of different thermopile detectors that can be placed within a small area, wherein each of the thermopile detectors would be responsive to a different range of wavelengths. That is, for small size detector packages (e.g., TO-5 size) or for arrays of detectors on a focal plane, providing a multiplicity of detectors responsive to different wavelengths is difficult or impossible using a conventional broadband detector with discrete optical filters.